Composite selenium electrode



p 30, 1947- A. 'voN HIPPEL Er AL 2, 8,0

' COMPOSITE SELENIUM ELECTRODE Filed Jam 5, 1943 2 Sheets-Sheet 2INVENTORS ARTHUR Von H/PREL (JAMES h! SCHULMANN Patented Sept. 30, 19472,428,055 COMPOSITE SELENIUM nnno'rnonn Arthur von Hippel, Weston, andJames H. Schulman, Cambridge, Mass, assignors, by mesne assignments, toFederal Telephone and Radio Corporation, New York, N. Y.,a'corporation-of Delaware Application January 5, 1943, Serial No.471,346

Claims.

This invention relates to electrodes and a method of: making them, of atype used in electrodeposition solutions and has for itsobject toprovide an electrode which can efficiently be used in a solution forelectro-depositing selenium.

:In the use of selenium plating arrangements in which metallic seleniumis deposited on a cathode, we have found (1) That unless the anodematerial consists of a substance inactive to the-electrolyte and to the:anodic products of electrolysis, the anode is attacked and destroyedeither by the chemical action of thereagents ofthe bath alone or by theelectro-chemical action when current is passed; and

(2) Even if the anode material does consist of such an inactivematerial, the. passage of current is attendedby a copious andundesirable evolution of gas at this anode and it also causes adepletion of the selenium content of the plating solution.

lheoretically these undesirable efiects might be eliminated by use of anelectrode of pure selenium in which case the passage of electric currentcauses anodic dissolution of the selenium rather than gas evolution, thedissolved selenium replenishing the concentration of that element in theplating bath as it is used up in the plating. Electrodes of seleniumalone, however, have such high electrical resistivitythat insufficientcurrent can be sent through the solution for a satisfactory plating.

We have discovered that by using electrodes in which selenium is admixedwith an inactive substance of good conductivity, there is little or no"bubbling at the electrode and the conductivity vention will be betterunderstood from the following detailed description taken in conjunctionwith the accompanying drawings, of which .Fig. 1 shows anelectro-deposition bath containing an electrode in accordance with thisin- ,vention Fig. 2'shows a cross-sectional view of an electrode. inaccordance with this invention;

Fig. 3 shows a cross-section view taken at line 33 of Fig. 2;

Fig. 4. shows a cross-sectional view of a modified form of electrode inaccordance with this invention;

Figs..5, 6 and'7 show alternative forms of electrodes.

Fig. 1 shows a container or jar l0 containing a selenium platingsolution I l which may be of a type disclosed in the copendingapplication of Von HiDPel and Bloom, Ser, No. 420,906, filed November29, 1941, which is now abandoned. The solution maybe made, for example,by dissolving selenium dioxide (SeO2) in large quantities in an aqueousbath which also contains an acid, for example sulpuric acid, H2SO4. Thepositive hydrogen ions from the acid tend to favor the production ofquadrivalent selenium cations by combining with they oxygen of theselenium dioxide andformin water. We have found that suchacids'assulphuric acid (H2SO4) selenic acid (HzSeOr), nitric acid(HNOS), and hydrochloric acid (HCl) are especially suitable forenhancing the concentration of quadrivalent selenium cations. Immersedinthe bath there is ananode l2 and a cathode H3. The cathode may be in theform of a suitable disc on which the deposition of selenium is to bemade at M, when a source of voltage I5 is connected between the cathodeand'the anode.

In accordance with the preferred form of this invention the anodeelectrode is constructed as shown in the cross-sectional views of Figs.2 and 3. An electrode I6 composed of a mixture of seleniumand carbon, isfitted with a suitable conducting wire or rod H, which may be one of thegroup platinum, gold or graphite, and which serves merely as amechanical holding device or handle'and as apoint of connection to thesource of current. The electrode I6 is preferably made of a mixture ofselenium powder and carbon powder, which may be-shaped in-a suitablemold. This mixture is heated to a temperature causing sintering orpreferably above the melting point of the selenium so that the seleniummelts and upon cooling forms a solid block throughout which the carbonis evenly distributed. The selenium is then preferably annealed byheating it up to a temperature kept below the melting point of seleniumto form the metallic or grayish form although this step is notessential. In such an arrangement the proportion of carbon to seleniumhas not been found critical as percentages of carbon in the mixturevarying from about to 75% by weight have been found satisfactory,

An alternative way of making the anode is to place the mixture ofselenium and carbon with or without simultaneous heating undersufficient pressure to produce an electrode of good mechanical strength.When such an arrangement is used it has been found that the percentageof carbon required for good electrical conductivity can be as low as2.5% by weight. When pressure without heat treatment is employed it hasbeen found that the anode tends to disintegrate more rapidly in thesolution than does the sintered or molten and annealed type anodedescribed above.

Fig. 4 shows a construction of the electrode which is somewhat differentfrom that shown in Fig. 2. In Fig. 4 the conducting wire or rod 'l'la.intrudes only a little way into the top of the electrode, instead ofalmost all the Way through it as shown inv Fig. 2. The arrangement ofFig. 4 has the advantage of enabling the electrod mixture "5a to bepractically entirely consumed without attacking the rod Ha. This may bedone by inserting the electrode into the bath up to a level which isbelow the bottom of the rod Ha. This is illustrated in Fig. 4 whereinthe dotted line 18 represents the surface of the bath.

The conductivity of the anode may be made to have any desired value byselection of the ratio of carbon to selenium and the proportions of theanode. For example, a mixture of 71.5% of selenium and 28.5% ofgraphite, by weight, formed into a rod-shaped electrode 4 inches longand it inch in diameter and fitted with a /1; inch diameter graphite rodas acore, has a resistance of about 1 ohm.

The efficiency of the anode in respect to the proportion of the currentflow consumed in forming the desired Se+ cations. depends somewhat onboth the proportions of the ingredients and on the current density atthe anode. For example, a mixture of 20% carbon and 80% selenium hasbeen found to have a current efficiency of about 90%, and the efiiciencyhas been found to change if the proportions and/or current density aregreatly varied from this.

By the use of an electrode in accordance with this construction, it hasbeen found that a fairly rapid deposition of the desired metallicgrayish form of selenium may be obtained on the cathode .13, with littleor none of the undesired bubbling of oxygen gas from the anode. Thecarbon, even though in relatively small proportions in the anode, servesto provide a high conductivity needed for therapid plating and theselenium passes into solution to replace that which is deposited on thecathode. Thus the solution is maintained at its desired concentration ofquadrivalent selenium.

It will be understood that while the invention has been described indetail with respect to the mixture of carbon with selenium, which is apreferred mixture in view of the cheapness of carbon, eithergold orplatinum or a mixture of both might be substituted for some or all ofthe carbon in the mixture in so far as electrical and chemical operationand efiiciency are concerned. Furthermore; the electrode is notnecessarily limited to use'as an anode as shown in the exampledescribed; it might in some instances be used as a cathode. For examplewhen used in a cathode in a dilute H2804 solution, hydrogen selenide(HzSe) will be generated which is useful in the anodicelectro-deposition of selenium. Such an anodic electro-deposition isdescribed in copending application Serial No. 467,562, filed December 1,1942, by Mortimer C. Bloom, now Patent No. 2,414,438, January 21, 1947.

Figs. 5, 6 and 7 show some different forms in which an electrodecontainin -selenium powder might be constructed. InFig. 5 the electrodematerial is cast or compressed and held within a porous cup 20 such as aporous ceramic cup within which there is inserted the rod H which ispreferably of carbon but could be one of the other inactive materialssuch as platinum or gold. The cast or compressed mixture of selenium andcarbon 2| is placed within the cup and the rod provides the connection.This may be used as an anode in place of that shown in Fig. 1. Theporosity of the ceramic cup allows the plating solution to come intocontact with the selenium.

In Fig. 6 the rod I! and the cast or compressed selenium substance 21 isplaced within a bag 22 of fiber glass so constructed that it is porousand allows theplating solution to enter into contact with the selenium.

Fig. '7 shows a modification in which the container for the selenium andthe conducting rod is a glass tube 2 3 having a porous end piece 24 madeof sintered glass which allows penetration of the solution to theselenium substance 21.

In each of the embodiments of Figs. 5, 6 and 7 the electrode materialmay either be cast directly within the porous container which may thenbe fitted with the connecting rod or wire, or the electrode material maybe cast first and then have the porous container loosely fitted over it.

The embodiments of Figs. 5, 6 and 7 has the advantage of keepingfloating particles from the electrode from filling up the bath. Inoperation, the carbon tends to fall off the electrode as it is used upand if these floating particles were allowed to float all over the'baththey would be objectionable. But by the use of the surrounding porouscontainer no such particles may get out into the bath.

What we claim is:

1. An electrode comprising a porous container, a conductor within thecontainer and a conductive solid body comprising coherent compressedselenium-carbon powder within the container around the conductor.

2. An electrode according to claim 1 in which the conductor is anon-reactive material of good conductivity from the group consisting ofplatinum, gold and graphite.

3. An electrode according to claim 1 in which the container is of fiberglass.

' 4. An electrode according to claim 1 in which the'porous container isof ceramic material.

5. An electrode according to claim 1 in which the porous containercomprises sintered glass.

ARTHUR VON HIPPEL.

JAMES H. SCHULMAN.

REFERENCES CITED The following references are of record in the file ofthis patent: 1

UNITED STATES PATENTS

